Within the Gulf of Aden, Djibouti is an important site where whale sharks (Rhincodon typus Smith 1828) aggregate seasonally; however, up to now, only few studies have been carried out in the area (Rowat et al. in Environ Biol Fishes, 2007. 10.1007/s10641-006-9148-7; Mar Freshw Res, 2011. 10.1071/MF10135; Rezzolla and Storai in Cybium 34:195-206, 2010). From 2015 to 2018, whale shark photo-id, skin biopsies, and zooplankton samples were collected to investigate the ecology of this species in Djibouti. Of the 190 photo-identified individuals, the majority were juvenile males, confirming a significant male-based aggregation (X-2 = 65, df = 1, p < 0.0001). Within each season, the maximum number of times an individual was re-sighted was 11, while the inter-annual re-sighting rates varied between 11 and 32%. Animals were feeding on surface zooplankton, consisting primarily of copepods (85%); however, Chaetognatha and Ctenophora appeared to be important taxa as well, since the areas most visited were also those with higher concentration of these organisms. Feeding zooplankton patches had a mean biomass of 42.2 +/- 31.9 mg m(-3), almost 1.5 higher than control stations, indicating that the abundance of this species was positively related to food availability. Furthermore, whale sharks exhibited a clear temporal distribution associated with the variation in zooplankton biomass due to the monsoon winds. Sharks aggregated in October-February, with a peak in sightings in November, and dispersed outside the upwelling season. delta C-13 values suggested that whale sharks might migrate to other habitats or shift their activities to deeper areas outside the aggregation phase. Little intra-specific variation in delta N-15 and delta C-13 was found between animals, suggesting a similar habitat use.
